I have seen two double barrel Gardner "machine guns", one with barrels shrouded (looks like a water jacket on machine guns), and one with both barrels exposed. I was allowed to shoot the open-barreled version about 1982 or so at Utah State University's Festival of the Old West.
I've heard of single barrel and five barrel Gardners but both are scarce...as if the twin barrel guns weren't already scarce enough.
Anyone have any more info on them?